Scientists have known for some time that ice shelves off West Antarctica are melting as deep, warm ocean waters eat at their undersides, but a new study shows that temperatures, and resultant melting, can vary far more than previously thought, within a time scale of a few years. The findings could have implications for estimates of future sea-level rise. The research was published this week in the journal Nature Geoscience.

Scientists studying seawater temperatures in the Amundsen Sea — host to ice shelves off the West Antarctic Ice Sheet — found a cycle of warming and cooling in the ocean over the 16 years of observations. They showed for the first time that while mass loss from the ice sheet increased during a warm period, it steadied and in some cases decreased during cooler phases. The authors found evidence that the changes are linked to El Niño-Southern Oscillation, a cyclic warming of the tropical Pacific Ocean that takes place about every 3 to 7 years.

Coauthor Pierre Dutrieux of Columbia University’s Lamont-Doherty Earth Observatory said the study “contributes an important foundation for predicting global sea-level rise. Our understanding of ice sheet-ocean interactions has progressed rapidly over the past decade. The seemingly immovable ice giants are actually very dynamic systems that respond quickly to a broad range of spatial and temporal changes in the ocean and the atmosphere.”

During eight Antarctic summers from 2000-2016, a team from the United Kingdom, the United States and South Korea used research vessels to observe changes in ocean temperature, salinity and currents near the Dotson Ice Shelf – an area of floating ice more than seven times the size of New York City. They found that temperature fluctuations in the relatively warm water in that area cause far greater changes in melting than occur along other parts of the Antarctic coastline where ocean temperatures are lower. It was the first time a complete multi-year cycle of ocean temperature changes and resulting changes in ice shelf melting have been documented in this region. Deep water in the Amundsen ranges from about 0.5 to 1 degree C (about 33 or 34 degrees F), while water in colder regions can go down to about minus 2 degrees C, or about 28 degrees F.

Adrian Jenkins, an oceanographer at British Antarctic Survey and lead author of the study, said, “[We] saw melt rates of Dotson Ice Shelf climb dramatically and then fall back. In future it will be critical to understand the duration and severity of the extremes in seawater temperature, whatever the cause, because we now see how quickly the glaciers respond to them.”

The melt rate at the base of the Dotson Ice Shelf was high between 2006-2009 (a warm period), and much lower in 2000 and 2012-2016 (both cool periods). In the recent cooler part of the cycle, outflow from the ice sheet slowed, and the sheet’s mass actually increased. This indicates that even on a time scale of a few years, the ice can respond if deep ocean temperatures drop below or rise above the average.

Coauthor Stan Jacobs, also from Lamont-Doherty, said, “This work confirms the theory that the ice sheet is sensitive to deep ocean temperatures. Further observations and a deeper understanding of what drives changes in those temperatures are critical.”

Although the study demonstrates that ice-shelf melting can vary widely in the short term, other recent research has confirmed that overall, West Antarctica is losing ice.

Scientists from the Korea Polar Research Institute, the University of Inha (Korea) and the Institute of Arctic and Alpine Research at the University of Colorado coauthored the research. The work was supported by the UK Natural Environment Research Council, the U.S. National Science Foundation and the Korea Polar Research Institute.